1. Technical Field of the Invention
The present invention relates to a probe type shape measuring sensor, and a NC processing equipment and a shape measuring method using the sensor.
2. Prior Art
When a workpiece must be manufactured precisely it is essential to have a technology for measuring the shape of the workpiece on the processing machine, i.e. so-called on-machine measurement technology. Such an on-machine measurement technology can improve the accuracy of the processing by eliminating the positioning errors which occur when the workpiece is removed and replaced, and at the same time the processing efficiency can be improved and the measurements can be automated and the manpower required for preparations can be saved.
Apparatus for measuring the shape of a workpiece, known in the prior art, include a probe type shape measuring sensor wherein the tip of a measurement probe touches the surface of the workpiece and measures the shape thereof. Such probe type shape measuring sensors are classified generally into analog and digital types depending on the means of detecting the position of the measurement probe.
In an analog-type shape measuring sensor, for example in an electric micrometer, displacements in the position of the probe are converted into analog electric values by detecting variations in the voltage of a differential transformer, electrostatic capacitance, resistance of a strain gauge, etc. However, because the system uses analog signals, large drifts in the output occur and the detected output is not very linear, so sub-micron accuracy such as about 0.1 xcexcm cannot possibly be obtained if the overall distance moved by the sensor is about 100 xcexcm.
Conversely, a digital-type shape measuring sensor, such as for example a digital micrometer, measures displacements in the position of the measurement probe, digitally using an optical scale, magnetic scale or a length measuring system using optical interferometer, therefore a maximum resolution of about 10 nm can be achieved.
However, even with the digital-type shape measuring sensor, the measurement probe must be supported by a linear ball bearing or an air slide, to enable it to move in the axial direction, and a spring or air pressure is used to press the probe onto the workpiece. Consequently, the measurement pressure fluctuates as the probe moves, and a large pressure is needed for the measurement and the pressure cannot be controlled freely, and this is a practical problem.
More explicitly, a spring applies a minimum pressure of about 10 grams which is too large to obtain a high accuracy, and because the spring force varies depending on the displacement of the probe resulting in variations in the pressure, there are large measurement errors, which is also another problem. When air pressure is applied, although the measurement pressure can be reduced by using a low air pressure, the minimum is still about 1 gram. There is another problem that if the air pressure is reduced, the stiffness of the air slide is also reduced, allowing the probe to tilt excessively, and the measurement errors are increased. Therefore, even with a digital system, a sub-micron accuracy of about 0.1 xcexcm cannot be achieved.
To obtain a high accuracy of the sub-micron order, it is desirable that the measurement pressure should be as low as possible (preferably, about 500 milligrams or less). And to prevent a deterioration in the measurement accuracy caused by sideways displacements of the probe during a measurement, the measurement pressure should preferably be freely adjustable. These requirements have been shown by analysis.
To satisfy these requirements, a high-accuracy shape measuring device typically as shown in FIG. 1 has been developed. This shape measuring device uses a minimum measuring force as small as about 50 milligrams, and measures the displacement of the measurement probe with a laser interferometer, thereby achieving a measurement accuracy of about 0.1 xcexcm. However, with the device shown in FIG. 1, many optical elements such as moving mirrors and prisms are required, so the device itself becomes very large and delicate, therefore the device has the problem that it cannot be installed on a processing machine for making measurements on the machine.
When the aforementioned probe type shape measuring sensor is installed on a conventional NC processing device, a personal computer etc. is used to output a command to define the position of each point to be measured, to the NC control device. And the probe is stopped for a predetermined time at the defined point, and when the position of the probe is considered to have stabilized, the output from the shape measuring sensor is saved to determine the shape of a workpiece. However, according to this means, the times required to move the probe to the defined positions and the waiting times during which the probe is stopped accumulate, a long time is required. In addtion as intermediate points between points cannot be measured, a large number of defined points are required, so causing the problem that very long time is required to complete the measurements.
The present invention is aimed at solving the various problems mentioned above. That is, an object of the present invention is to provide a probe type shape measuring sensor with a small electric drift, excellent linearity of the output, small variations in measurement pressures during changes in the position of the probe, without decreasing the stiffness of the probe bearing, measurement pressures that can be adjusted to a constant very small load and changed freely, thus a sub-micron accuracy of about 0.1 xcexcm can be obtained, and also capable of being made compact, and easily applied to on-machine measurements, and an NC processing apparatus and a shape measuring method using the sensor.
Another object of the present invention is to offer an NC processing apparatus and a shape measuring method using the aforementioned probe type shape measuring sensor, in which the waiting time is reduced, and the shape between defined points can be measured, thereby enabling the number of necessary defined points to be reduced and the measuring time shortened.
According to the present invention, a probe type shape measuring sensor is provided and characterized to be composed of a probe head (10) that supports a probe (2) that contacts a workpiece (1) in such a way that the probe can move towards the workpiece with an extremely low resistance to sliding and drives the probe towards the workpiece with a very low force, and a displacement measuring device (20) that measures the displacement of the probe, very accuracy and without contact.
Because the probe (2) is supported by the probe head (10) so that it can move with an extremely low resistance to sliding and is driven towards the workpiece, the probe can trace the surface of the workpiece precisely while contacting the surface of the workpiece with a very low load (about 500 mgf or less). Furthermore, by measuring the displacement of the probe with the displacement measuring device (20) which is very accurate and requires no contact, a sub-micron accuracy of about 0.1 xcexcm can be achieved.
According to a preferred embodiment of the present invention, the aforementioned probe head (10) is provided with a long thin probe shaft (12) with the probe installed at one end (12a) thereof and a step in cross section (11a, 11b) at an intermediate portion thereof, air bearing (14a, 14b) that are disposed at each side of the above-mentioned step and support the probe shaft, and a means (16) of feeding air that supplies gas at the first pressure to the location of the aforementioned step; the above-mentioned air bearings have a high stiffness in the radial direction and are disposed in such a way that the gas at the first pressure causes the probe shaft to float to reduce its resistance to sliding; the aforementioned gas feeding means keeps the pressure or pressures of gas or gasses at second and/or third pressures supplied to the location of the aforementioned step at a constant value or constant values, thereby the gas feeding means produces a driving force due to the step in the direction of the workpiece being measured and keeps the load very small within a predetermined range.
The long thin probe shaft (12) with the probe attached at one end (12a) thereof is supported by the air bearings (14a, 14b), and the first pressurizing gas for instance, compressed air increases the stiffness of these air bearings in the radial direction, and causes the probe shaft to float. Thus the probe shaft can be supported with an extremely low resistance to sliding and can move towards the workpiece while the probe shaft is prevented from being tilted by the friction between the probe and the workpiece, therefore, measurement errors can be prevented from increasing. In addition, the steps (11a, 11b) are constructed at intermediate portions of the probe shaft, and the means (16a, 16b) of feeding gas supplies the location of the steps with second and/or third pressurizing gas or gasses (for instance, another sources of compressed air). Thereby the driving force produced by the difference in sectional areas of the shaft in the direction of the workpiece can be maintained at a constant very low load within a predetermined range.
Therefore, because the driving force is created by the difference in sectional areas due to the step in the shaft in the direction of the workpiece, a drift of the output is completely eliminated. Also, because second and/or third pressurizing gas or gasses are supplied to produce the driving force in the direction of the workpiece, independently from the first pressurizing gas that keeps the probe shaft floating, the measuring pressure can be adjusted to a constant very small load without degrading the stiffness of the bearings of the probe. Furthermore, as the driving force in the direction of the workpiece is proportional to the pressure difference in shaft cross sectional areas and no springs etc. are used, variations in the measuring force caused by changes in the position of the probe can be eliminated, so that the linearity of the output is improved, and the measuring force can be freely changed by controlling the pressures of second and/or third pressurizing gas or gasses.
The aforementioned driving force given to the probe shaft in the direction of the work piece should preferably be about 10 mgf or more and no more than about 500 mgf. If the driving force in the direction of the workpiece exceeds about 500 mgf, friction between the probe and the workpiece increases, resulting in a large tilt of the probe shaft, so a sub-micron accuracy of about 0.1 xcexcm cannot be obtained. Also if it is less than about 10 mgf, the probe may often bounce, so that the measuring speed is greatly reduced.
The above-mentioned displacement measuring device (20) is provided with a reflecting mirror (21) installed at the other end (12b) of the probe shaft, an optical fiber (22) with its emitting end surface (22a) located opposite and apart from the aforementioned reflecting mirror, and a laser interferometric displacement meter (24) that transmits laser light through the above-mentioned optical fiber to the aforementioned reflecting mirror and measures the position of the reflecting mirror by light reflected between the reflecting mirror and the emitting end surface.
In this configuration, using the laser interferometric displacement meter (24), the position of the reflecting mirror (21) can be measured with a high accuracy of 0.1 xcexcm. In addition, because the laser light is transmitted to the reflecting mirror through the optical fiber (22), the moving parts of the probe head (10) and the laser interferometric displacement meter (24) can be made compact. Moreover, because the probe shaft (12) can be made with a light weight, the response time for measurements is reduced, so high-speed measurements can be achieved.
Also because the main unit of the laser interferometric displacement meter can be located away from the probe head, the measuring instrument can be protected from thermal distortions so that highly accurate measurements can be achieved.
According to another aspect of the present invention, the invention is provided with NC processing equipment, that incorporates the aforementioned probe type shape measuring sensor, and moves the sensor by a numerical control system relative to the workpiece, thereby measuring the shape of the workpiece without needing to remove the workpiece which is to be processed henceforth.
This configuration enables a sub-micron accuracy of about 0.1 xcexcm to be achieved, and because the probe type shape measuring sensor that can be made compact and is installed on the NC processing equipment, on-machine measurements become possible, positioning errors that may otherwise occur when the workpiece is removed and remounted can be eliminated so improving the accuracy of the processing, and the time and manpower required for preparations can be saved and the processing efficiency can be improved by automation.
According to still another embodiment of the present invention, the aforementioned NC processing equipment is provided with an interface that outputs the coordinates of each numerical control axis and the signals from the probe type shape measuring sensor, in real time for use outside the equipment.
Using this configuration, the coordinates of each numerical control axis and the output signals from the probe type shape measuring sensor can be saved in a computer etc. outside the equipment, through the interface, in real time while the probe type shape measuring sensor is being driven and moved to obtain a profile of the workpiece, without needing to stop the NC control equipment at a defined point for positioning. Therefore, the number of defined points can be reduced and the time required for the measurements is decreased.
Further according to the present invention, the probe (2) in contact with the workpiece (1) is installed at one end (12a) of the long thin probe shaft (12) which has steps (11a, 11b), and while the aforementioned probe shaft is supported by the first pressurizing gas so as to be able to move with an extremely low friction, the shaft is maintained with a high stiffness in the radial direction, second and/or third pressurizing gas or gasses are supplied to the location of the above-mentioned steps, the driving force of the probe shaft in the direction of the workpiece is kept very small by the pressure or pressures thereof, and the displacement of the probe in the direction of the workpiece is measured with the laser interferometric displacement meter (24).
According to this method, the probe (2) is supported so it can move with an extremely low resistance to sliding, and is driven in the direction of the workpiece. Thereby the probe can precisely follow the profile of the surface of the workpiece while the probe is kept in contact with the surface of the workpiece with a very small load. In addition, by measuring the displacement of the probe with the laser interferometric displacement meter (24) with a high accuracy in a manner which requires no contact, a sub-micron accuracy of about 0.1 xcexcm can be achieved. Independently from the first pressurizing gas that makes the probe shaft float, second and/or third pressurizing gas or gasses are supplied that produce the driving forces in the direction of the workpiece. Therefore, the measuring pressure can be adjusted to maintain a constant, very small load without reducing the stiffness of the bearings of the probe. In addition, because the driving force in the direction of the workpiece is proportional to the pressure applied to the portion where there is a step, variations in the measuring pressure that might otherwise result from changes in the position of the probe can be eliminated, so that the linearity of the output is improved, and furthermore, the measuring pressure can be changed freely by controlling the pressure or pressures of the second and/or third pressurizing gas or gasses.
Moreover, the present invention also provides a shape measuring method wherein the above-mentioned probe type shape measuring sensor is built into an NC processing device, and the sensor is moved relative to the workpiece by a numerical control system, and thus the shape of the workpiece after processing can be measured without removing it from the equipment.
This method makes on-machine measurement possible, eliminates positioning errors due to removing and remounting the workpiece, thereby increasing the accuracy of processing, and furthermore, the manpower required for preparation at that time can be saved, thereby improving the efficiency of processing and, at the same time the operation can be automated.
In the aforementioned shape measuring method, it is preferred that the above-mentioned NC processing equipment is not stopped but is used to measure the shape of the workpiece by directly outputting in real time the coordinates the numerical control axes of the aforementioned NC processing equipment together with the output signals from the above-mentioned probe type shape measuring sensor to external equipment.
Using this method, the coordinates of the numerical control axes of the NC processing device and the output signals from the probe type shape measuring sensor can be stored in a computer etc. outside the equipment, in real time as the probe type shape measuring sensor is being used to obtain a profile of the workpiece. Therefore, the shape of the workpiece can be measured, and the measuring time can be reduced without needing to stop the NC processing device.